Stabilities and structures of β- and α-Sn nanoparticles are studied using density functional theory. Results show that β-Sn nanoparticles are more stable. For both phases of Sn, nanoparticles smaller than 1 nm (∼48 atoms) are amorphous and have a band gap between 0.4 and 0.7 eV. The formation of band gap is found to be due to amorphization. By increasing the size of Sn nanoparticles (1–2.4 nm), the degree of crystallization increases and the band gap decreases. In these cases, structures of the core of nanoparticles are bulk-like, but structures of surfaces on the faces undergo reconstruction. This study suggests a strong size dependence of electronic and atomic structures for Sn nanoparticle anodes in Li-ion batteries.
In this study we investigate the magnetic exchange coupling behavior in MnBi/FeCo system at the hard/soft interface. Exchange spring MnBi/FexCo1−x (x = 0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited onto quartz glass substrates in a DC magnetron sputtering unit from alloy targets. According to magnetic measurements, using a Co-rich layer leads to more coherent exchange coupling with optimum soft layer thickness of about 1 nm. In order to take into account the effect of structural factors at the hard/soft interface which can deteriorate the exchange coupling for thicker soft magnetic layers, we have combined cross-sectional High Resolution Transmission Electron Microscopy (HR-TEM) analysis with DFT calculations and micromagnetic simulations. DFT calculations predict formation of a polycrystalline FeCo layer with coexisting crystalline (110) and disordered (randomly-oriented) phases which is confirmed by HR-TEM images. Moreover, our micromagnetic simulations show how the thickness of the FeCo layer and the interface roughness between the hard and soft magnetic phases both control the effectiveness of exchange coupling in MnBi/FeCo system. Our method can be applied to study other exchange spring systems.
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